Fix segfault on s3 filesystem (#1829)

* fix error

* apply clang-format

tensorflow_io/core/filesystems/s3/s3_filesystem.cc

@@ -143,13 +143,15 @@ static Aws::Client::ClientConfiguration& GetDefaultClientConfig() {
     }
     // if these timeouts are low, you may see an error when
     // uploading/downloading large files: Unable to connect to endpoint
+    const char *connect_timeout = getenv("S3_CONNECT_TIMEOUT_MSEC"),
+               *request_timeout = getenv("S3_REQUEST_TIMEOUT_MSEC");
     int64_t timeout;
     cfg.connectTimeoutMs =
-        absl::SimpleAtoi(getenv("S3_CONNECT_TIMEOUT_MSEC"), &timeout)
+        connect_timeout && absl::SimpleAtoi(connect_timeout, &timeout)
             ? timeout
             : kS3TimeoutMsec;
     cfg.requestTimeoutMs =
-        absl::SimpleAtoi(getenv("S3_REQUEST_TIMEOUT_MSEC"), &timeout)
+        request_timeout && absl::SimpleAtoi(request_timeout, &timeout)
             ? timeout
             : kS3TimeoutMsec;
     const char* ca_file = getenv("S3_CA_FILE");
@@ -189,8 +191,11 @@ static void GetS3Client(tf_s3_filesystem::S3File* s3_file) {
           }
         });
 
+    const char* disable_multi_part_download =
+        getenv("S3_DISABLE_MULTI_PART_DOWNLOAD");
     int temp_value;
-    if (absl::SimpleAtoi(getenv("S3_DISABLE_MULTI_PART_DOWNLOAD"), &temp_value))
+    if (disable_multi_part_download &&
+        absl::SimpleAtoi(disable_multi_part_download, &temp_value))
       s3_file->use_multi_part_download = (temp_value != 1);
 
     const char* endpoint = getenv("S3_ENDPOINT");
@@ -220,12 +225,15 @@ static void GetTransferManager(
   if (s3_file->transfer_managers.count(direction) == 0) {
     uint64_t temp_value;
     if (direction == Aws::Transfer::TransferDirection::UPLOAD) {
-      if (!absl::SimpleAtoi(getenv("S3_MULTI_PART_UPLOAD_CHUNK_SIZE"),
-                            &temp_value))
+      const char* upload_chunk_size = getenv("S3_MULTI_PART_UPLOAD_CHUNK_SIZE");
+      if (upload_chunk_size == nullptr ||
+          !absl::SimpleAtoi(upload_chunk_size, &temp_value))
         temp_value = kS3MultiPartUploadChunkSize;
     } else if (direction == Aws::Transfer::TransferDirection::DOWNLOAD) {
-      if (!absl::SimpleAtoi(getenv("S3_MULTI_PART_DOWNLOAD_CHUNK_SIZE"),
-                            &temp_value))
+      const char* download_chunk_size =
+          getenv("S3_MULTI_PART_DOWNLOAD_CHUNK_SIZE");
+      if (download_chunk_size == nullptr ||
+          !absl::SimpleAtoi(download_chunk_size, &temp_value))
         temp_value = kS3MultiPartDownloadChunkSize;
     }
     s3_file->multi_part_chunk_sizes.emplace(direction, temp_value);
@@ -1015,7 +1023,7 @@ void CreateDir(const TF_Filesystem* filesystem, const char* path,
 
   PathExists(filesystem, dir_path.c_str(), status);
   if (TF_GetCode(status) == TF_OK) {
-    std::unique_ptr<TF_WritableFile, void (*)(TF_WritableFile * file)> file(
+    std::unique_ptr<TF_WritableFile, void (*)(TF_WritableFile* file)> file(
         new TF_WritableFile, [](TF_WritableFile* file) {
           if (file != nullptr) {
             if (file->plugin_file != nullptr) tf_writable_file::Cleanup(file);